Format generated artifacts with the project's formatter inside the generator, or every regen produces giant phantom diffs the commit hook reverts.
If a code generator writes an artifact (e.g. a 600KB openapi.json via Python json.dump(indent=2)) but a pre-commit prettier hook reformats that file type, every regeneration shows a massive phantom diff (~60k lines in our case) that the hook silently reverts at commit time. Symptoms that waste investigation time: git status shows the generated file modified after every regen; staging it and committing makes the change vanish (post-prettier it's byte-identical to HEAD); agents and humans repeatedly re-diff the file trying to find the 'real' change.
Fix: run the project formatter on generated outputs at generation time, in the generator itself (e.g. add the JSON file to the existing prettier --write invocation that already covers the generated TypeScript). Acceptance test: a no-op regeneration must leave the artifact byte-identical to HEAD. Apply the same rule to anything the generator appends after formatting (trailing exported consts etc.) — emit them in formatter-stable style (quote style, semicolons) or they'll churn on the next commit instead.